Pickling of steel



Patented Nov. 7', 193% l ama STATES gimi lPlICKLlING OF STEEL Donald Cleve lHiilty, Apollo, Pa.

No Drawing. Application December 29, 1937, Serial No. 182,321

2 Claims.

This invention relates to the pickling of steel and is particularly concerned with the removal of red oxide from silicon steel. In explanation, silicon steel sheets, upon being annealed, develop 5 a reddish surface color in addition to the usual scale to be expected. Ordinary pickling does not remove this red color.

According to the present invention, a pickling bath is prepared which contains, in addition to the usual concentration of sulphuric acid, sulphur dioxide to the extent of at least .025 per cent. by weight of the solution. In some instances hydrochloric acid may be added to accelerate the scale removal. Other pickling additions may be made, such as that of the various inhibitors which have become common. In-

case it is desired to remove the reddish color without also removing the scale, the acid concentration may be kept to a very low value, bleaching occurring if the sulphur dioxide concentration is above the factor mentioned.

The objectionable reddish tinge is removed from annealed silicon sheets upon immersion in either of the above solutions. In the case of a solution having a. normal acid concentration? common to pickling liquors, both the reddish color and ordinary scale will be removed. In the case of certain types of silicon sheets the scale re mains, and in such instances the solution containing a low concentration of acid but including the sulphur dioxide functions to bleach the reddish color to a uniform grayish tint. It has been found that here is an abrupt decrease in the solubility of sulphur dioxide gas in acid pickling solu- 35 tion' as the temperature of the solution is raised above 140 degrees Fahrenheit. ,a

As a specific example, the removal of both ordinary scale and red oxide from annealed silicon steel sheets may be effected by immersing the sheets for ten minutes in an ordinary 7 per cent, sulphuric acid pickling solution containing, however, .025 per cent. or more by weight of sulphur I dioxide, with the solution at a temperature not exceeding 140 degrees Fahrenheit. The scale removing action of the solution may be accelerated by the addition of a small amount of hydrochloric acid. The sulphur dioxide concentration may be maintained by direct injection of the gas or by the use of suitable compounds which release this 5 gas upon reaction with the pickling solution. Examples of such compounds are sodium sulphite, sodium bi-sulphite, or sodium thiosulphate.

In case bleaching alone is desired the pickling solution used should not contain more" than 1.0 0 per cent. by weight of sulphuric acid.- No hydrochloric acid should be used. Immersion of the sheets for five minutes at a temperature of 140 degrees Fahrenheit in such a picklingsolution including, of course, the proper concentration of 15 sulphur dioxide, will remove the red color without materially affecting the normal scale. Preferably, a suitable inhibitor should be added to the solution.

I claim:.

1. A process of removing the red color from heat treated silicon steel without substantial scale removal, including subjecting the steel to the action of a slightly acid solution containing sulphur dioxide while maintaining the solution 25 at temperatures not exceeding 140 degrees Fahrenheit, the sulphur dioxide being maintained at a concentration sufficient to efiect removal of the red color and the acidity of the solution being sufilciently low to prevent substantial scale 30 removal.

2. A process of removing the red color from heat treated silicon steel without substantial scale removal, including subjecting the steel to the action of a slightly acid solution containing sulphur dioxide while maintaining the solution at temperatures not exceeding 140 degrees Fahrenheit, the sulphur dioxide being maintained at a concentration in excess of .025v per cent by weight of the solution to. effect removal of the red color 40 and the acidity of. the solution being sufllciently low to prevent substantial scale removal.

DONALl) CLEVE HILTY. 

